std::variant_size, std::variant_size_v
Defined in header <variant>
.
// 1)
template <class T>
struct variant_size; /* undefined */ // (since C++17)
// 2)
template <class... Types>
struct variant_size<std::variant<Types...>>
: std::integral_constant<std::size_t, sizeof...(Types)> { };// (since C++17)
// 3)
template <class T> class variant_size<const T>; // (since C++17)
// 3)
template <class T> class variant_size<volatile T>;
template <class T> class variant_size<const volatile T>; // (since C++17, deprecated in C++20)
Provides access to the number of alternatives in a possibly cv-qualified variant as a compile-time constant expression.
Formally,
meets the UnaryTypeTrait requirements with a base characteristic of std::integral_constant<std::size_t, sizeof...(Types)>
meets the UnaryTypeTrait requirements with a base characteristic of std::integral_constant<std::size_t, variant_size<T>>
Helper variable template
template <class T>
inline constexpr std::size_t variant_size_v = std::variant_size<T>::value; // (since C++17)
Inherited from std::integral_constant
Member constants
pub | value [static] | sizeof...(Types) (public static member constant) |
Member functions
pub | operator std::size_t | converts the object to std::size_t, returns value (public member function) |
pub | operator()(C++14) | returns value (public member function) |
Member types
|Type|Definition|
|::|::|
|value_type|std::size_t
|
|type|std::integral_constant<std::size_t, value>
|
Notes
All specializations of std::variant_size
satisfy UnaryTypeTrait with base characteristic std::integral_constant<std::size_t, N>
for some N
.
Example
#include <any>
#include <cstdio>
#include <variant>
static_assert(std::variant_size_v<std::variant<>> == 0);
static_assert(std::variant_size_v<std::variant<int>> == 1);
static_assert(std::variant_size_v<std::variant<int, int>> == 2);
static_assert(std::variant_size_v<std::variant<int, int, int>> == 3);
static_assert(std::variant_size_v<std::variant<int, float, double>> == 3);
static_assert(std::variant_size_v<std::variant<std::monostate, void>> == 2);
static_assert(std::variant_size_v<std::variant<const int, const float>> == 2);
static_assert(std::variant_size_v<std::variant<std::variant<std::any>>> == 1);
int main() { std::puts("All static assertions passed."); }
All static assertions passed.